This invention relates to the monitoring of heart rates, and more particularly to a heart rate monitoring method and cardiotachometer capable of monitoring heart rate under difficult or adverse conditions, such as during physical exercise.
Cardiotachometers are used in a wide variety of situations and include a wide variety of types of instruments. The most common uses are in medical related applications for diagnostic and rehabilitation monitoring and in recreational exercise monitoring. Most cardiotachometers available heretofore have performed satisfactorily under the static conditions of diagnostic and rehabilitation monitoring. However, the only ones capable of providing reliable monitoring under the stringent and noisy conditions of physical exercise require the use of chest electrodes. These are uncomfortable and obtrusive and they restrict the range of activity.
A cardiotachometer is made up of three general sections: 1) Transducer; 2) Processing; 3) Readout. A transducer or conductor is attached to a person in a variety of ways and provides an electric output signal. This signal is passed to electronic circuitry which amplifies, filters, detects, counts and provides an electrical output signal representing some measurement of the heart rate. This representation generally is provided as a visual display or auditory signal.
The most common form of transducer, or heartbeat pick-up, is of the chest electrode type. Medical literature has described in great detail the common chest electrode in terms of materials, placement, and other variables. The number of chest electrodes and placement strategy have been defined for some time. The most common variation on the chest electrode is simply to use the electrode as a pick-up on some part of the body other than the chest. Several devices have used electrode pick-ups to sense heart rate related signals through the hands or fingers. These types of electrodes are typically metal buttons of handgrip rings, Common configurations have two electrodes on each hand, or two electrodes on one hand and one electrode in the other hand.
In all cases of the electrode pick-up, electrical signals are detected by making direct contact with skin by the electrode. In many of the chest electrode applications, conductive cream is added between the electrode and the skin to increase conductivity. In exercise applications, chest electrodes are typically held in place by elastic straps that encircle the chest. Alternatively, the electrodes are held in place with tape or adhesive pads surrounding the electrode.
Electrodes are designed to conduct the energy generated by the heart muscle to the inputs of an amplifier. Depending on the placement and configuration of the chest electrodes, the amplitude of the signal varies from 0.1 to 4.0 millivolts. Electrodes used on other parts of the body, such as the hands, result in even smaller and more variable amplitudes. Hand held or finger contact electrodes show much lower amplitude because of the extremity from the heart. In addition, hand held or finger contact show greater variability in conductivity due to contact pressure variations and contact surface contaminants.
Another common method of sensing heart rate uses an infrared light source and detector. This is commonly referred to as a plethysmograph transducer. Blood volume changes associated with heart rate are detected at the extremities by this method. The most common locations for such a transducer are either the fingers or the ear lobe. A clip containing the infrared light source and infrared detector is attached over the extremity. The infrared signal travels through the finger or ear lobe and is sensed by the detector. Variations in blood density in the extremity affect the amount of light passing and thus received by the detector. The detector output is connected to circuitry that amplifies, filters, and compares the signal to an amplitude criteria, counts the "hits" and displays the counts.
A piezoelectric transducer converts mechanical force to electrical energy. This method has been used to detect heart rate at the fingers, and employs a very thin piezoelectric crystal which rests against the finger. Blood volume changes create minute expansions and contractions in the finger, flexing the piezoelectric crystal. The electrical signals generated by movements of the piezoelectric crystal are amplified, filtered, measured against an amplitude criteria, counted and displayed.
Piezoelectric transducers are particularly susceptible to vibration noise. Infrared pick-ups readily respond to mechanical artifacts such as pressure changes. The exercise environment is predominated by both vibratory noise and pressure changes. As a result, these transducer types are at a disadvantage in an exercise application.
Chest electrodes are minimally susceptible to vibratory noise and pressure changes. However, they are generally excluded for use in recreational monitoring because of the logistics involved in their attachment and placement and their obtrusiveness to the user. Electrodes such as wrist straps or hand grips avoid most of the mechanical susceptibility of piezo and infrared pick-ups, but are affected by potential changes generated by voluntary muscle activity and static electricity.
The foregoing illustrates why cardiotachometers available heretofore have not been completely satisfactory in monitoring heartrates during physical exercise because of obtrusiveness or interfering noise.
The general concepts and problems associated with conventional electrocardiography are described in such publications as "Electrocardiology Made Easy" by D. M. Van Wynsberghe and Ronald E. Hammond, in Carolina Tips, pages 9-11, volume 47, No. 3, Mar. 1, 1984 of Carolina Biological Supply Company; "R-wave Detection In The Presence Of Muscle Artifacts", by Olivier Y. De Vel, IEEE Transactions On Biomedical Engineering, pages 715-717, Volume BME-31, No. 11, November 1984; and "Estimation Of QRS Complex Power Spectra For Design Of A QRS Filter", by Nitish V. Thakor, John G. Webster and Willis J. Tompkins, IEEE Transactions On Biomedical Engineering, Pages 702-705, Volume BME-31, No. 11, November 1984.
Typical types of cardiotachometers are disclosed in U.S. Pat. Nos. 4,592,367; 4,616,659; and 4,667,682.